Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for deghosting seismic data.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of reservoirs. Thus, providing a high-resolution subsurface image is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, a seismic survey system 100, as shown in FIG. 1, includes a vessel 102 that tows plural receivers 104 distributed along a streamer 106. Vessel 102 may tow plural streamers 106 at the same time. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to the ocean surface 110. Also, the plural streamers 106 may form a constant angle (i.e., the streamers may be slanted) with respect to the ocean surface as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference.
Still with reference to FIG. 1, each streamer may have a head float 106a and a tail float 106b connected to respective streamer ends for maintaining the given depth z1. A front-end gear 112 that includes various cables connects streamers 106 to vessel 102. Vessel 102 also tows a sound source 120 configured to generate an acoustic wave 122a. Acoustic wave 122a propagates downward and penetrates the seafloor 124, eventually being reflected by a reflecting structure 126 (reflector). The reflected acoustic wave 122b propagates upward and is detected by detector 104. For simplicity, FIG. 1 shows only one path 122a corresponding to the acoustic wave. However, the acoustic wave emitted by the source 120 may be substantially spherical, e.g., it propagates in all directions starting from source 120. Parts of the reflected acoustic wave 122b (primary) are recorded by the various detectors 104 (the recorded signals are called traces) while parts 122c of the reflected wave 122b pass the detectors 104 and arrive at the water surface 110. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic waves), reflected wave 122c is reflected back toward another detector 104 as shown by wave 122d in FIG. 1. Wave 122d is normally referred to as a ghost wave because it is due to a spurious reflection. Ghosts are also recorded by detectors 104, but with a reverse polarity and a time lag relative to primary wave 122b. The degenerative effect that the ghost arrival has on seismic bandwidth and resolution are known. In essence, interference between primary and ghost arrivals causes notches, or gaps, in the frequency content recorded by the detectors, which reduces the useful bandwidth.
The recorded traces may be used to determine the subsurface (i.e., earth structure below surface 124) and to determine the position and presence of reflectors 126. However, ghosts disturb the final subsurface image's accuracy and for at least this reason, various methods exist for removing ghosts, i.e., deghosting, from recorded seismic data.
Such methods are described in U.S. Pat. Nos. 4,353,121 and 4,992,992 (the entire content of which are incorporated herein by reference) but they are seismic processing procedures in one dimension and in two dimensions. Such procedures, however, cannot be generalized to three dimensions. This is so because a sampling interval of the sensors in the third dimension is given by the separation between the streamers, on the order of 150 m, which is much larger than the sensors' sampling interval along the streamers, which is on the order of 12.5 m. Also, existing procedures may apply a deghosting step at the beginning of processing, which is not always very efficient.
Improved deghosting methods are described in U.S. patent application Ser. No. 13/155,778, filed Jun. 8, 2011, and being assigned to the assignee of the present disclosure. The entire content of this application is incorporated herein by reference.
However, there is still a need to provide systems and methods that are more efficient than the existing methods.